In the design of stellarators as plasma confinement devices and candidates for fusion reactors, simplifying assumptions concerning basic MHD equilibrium have been made. In previous analyses of helical axis stellarators, the assumption has been made that for large aspect ratio, the stellarator could be approximated by an infinite cylinder. This assumption reduced the MHD equilibrium equations to two-dimensions, thus affording simplified solutions. However, during experiments on such stellarators, plasma confinement was lost at high plasma pressure, contrary to theoretical predictions based on two-dimensional equilibrium solutions.
The inventors have recently determined that in a three-dimensional MHD equilibrium, the diamagnetic and Pfirsch-Schl/u/ ter currents driven by the pressure on any given flux surface may resonate with the rotational transform of a flux surface elsewhere in the plasma. This results in the appearance of magnetic islands and the destruction of flux surfaces in the equilibrium. These resonant equilibrium currents are unique to three-dimensional equilibria and are precluded by symmetry in one or two dimensions. In one- or two-dimensional equilibria islands may be generated by the appearance of a (symmetry breaking) tearing instability. However, the islands driven by resonant diamagnetic and Pfirsch-Schl/u/ ter currents are intrinsic to the equilibrium. When these islands are sufficiently large that they overlap, the flux surfaces are destroyed, and there is no equilibrium.
Therefore, it is an object of the present invention to provide a method and apparatus for maintaining three-dimensional MHD equilibrium in helical axis stellarators.
Additional objects, advantages, and novel features of the invention will be set forth in part in the description which follows, and in part will become apparent to those skilled in the art upon examination of the following or may be learned by practice of the invention.